1. Field of the Invention
The present invention relates to a surface light source using light emitting diodes (LEDs), and more particularly, to a surface light source that can solve the problems, which may be caused by the increase of a driving voltage and current, by electrically interconnecting a plurality of LEDs using the combination of a series connection and a parallel connection and a backlight unit for a liquid crystal display (LCD), which has the surface light source.
2. Description of the Related Art
A cold cathode fluorescent lamp (CCFL) is widely used as a light source of a conventional backlight for an LCD. Since the CCFL uses mercury gas, it may cause the environmental pollution. Furthermore, the CCFL has a relatively slow response time and a relatively low color reproduction. In addition, the CCFL is not proper to reduce the weight, thickness and overall volume of an LCD panel to which it is applied.
However, LEDs are eco-oriented and have a response time of several nano seconds, thereby being effect for a video signal stream and enabling an impulsive driving. Furthermore, the LEDs have 100% color reproduction and can properly vary luminance and color temperature by adjusting a quantity of light emitted from red, green and blue LEDs. In addition, the LEDs are proper to reduce the weight, thickness and overall volume of the LCD panel. Therefore, in recent years, they have been widely used as a light source of a backlight unit for the LCD.
The LCD backlight employing the LEDs can be classified into an edge type backlight and a direct type backlight according to positions of the light source. In the edge type backlight, the light source is positioned at a side and emits light toward a front surface of the LCD panel using a light guide plate. In the direct type backlight, the light source is a surface light source placed under the LCD panel and having a surface area almost identical to that of the LCD panel and directly emits light toward the front surface of the LCD panel.
FIG. 1 shows a conventional surface light source used in a backlight unit of a direct type LCD panel. As shown in FIG. 1, a conventional surface light source used in a direction type LCD panel includes bar-shaped substrates 110. A plurality of red, green and blue LEDs are arranged on each of the bar-shaped substrate 110. The bar-shaped substrates 110 are arrayed in a direction. That is, the plurality of red, green and blue LEDs 121, 122 and 123 are arranged on each of the bar-shaped substrates 110, in which the identical color LEDs are interconnected in series and directly connected to a corresponding color LED driving circuit 221, 222, or 223 of an LED driving unit 22. In the conventional surface light source, one LED driving unit 22 is required for each bar-shaped substrate. In addition, each LED driving unit 22 requires three different driving circuits 121, 122 and 123.
In the LED connection structure of the conventional surface light source, the LEDs are interconnected in series in each driving circuit. However, when the large number of LEDs is interconnected in series, a very high driving voltage is necessary.
However, since the increase of the driving voltage or driving current is limited due to a structural limitation of an integrate circuit used in the driving circuit or other design limitations, it is impossible to apply the conventional LED connection structure to a surface light source using the large number of LEDs.